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TOMOYO Linux Cross Reference
Linux/fs/ubifs/budget.c

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  1 /*
  2  * This file is part of UBIFS.
  3  *
  4  * Copyright (C) 2006-2008 Nokia Corporation.
  5  *
  6  * This program is free software; you can redistribute it and/or modify it
  7  * under the terms of the GNU General Public License version 2 as published by
  8  * the Free Software Foundation.
  9  *
 10  * This program is distributed in the hope that it will be useful, but WITHOUT
 11  * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
 12  * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License for
 13  * more details.
 14  *
 15  * You should have received a copy of the GNU General Public License along with
 16  * this program; if not, write to the Free Software Foundation, Inc., 51
 17  * Franklin St, Fifth Floor, Boston, MA 02110-1301 USA
 18  *
 19  * Authors: Adrian Hunter
 20  *          Artem Bityutskiy (Битюцкий Артём)
 21  */
 22 
 23 /*
 24  * This file implements the budgeting sub-system which is responsible for UBIFS
 25  * space management.
 26  *
 27  * Factors such as compression, wasted space at the ends of LEBs, space in other
 28  * journal heads, the effect of updates on the index, and so on, make it
 29  * impossible to accurately predict the amount of space needed. Consequently
 30  * approximations are used.
 31  */
 32 
 33 #include "ubifs.h"
 34 #include <linux/writeback.h>
 35 #include <linux/math64.h>
 36 
 37 /*
 38  * When pessimistic budget calculations say that there is no enough space,
 39  * UBIFS starts writing back dirty inodes and pages, doing garbage collection,
 40  * or committing. The below constant defines maximum number of times UBIFS
 41  * repeats the operations.
 42  */
 43 #define MAX_MKSPC_RETRIES 3
 44 
 45 /*
 46  * The below constant defines amount of dirty pages which should be written
 47  * back at when trying to shrink the liability.
 48  */
 49 #define NR_TO_WRITE 16
 50 
 51 /**
 52  * shrink_liability - write-back some dirty pages/inodes.
 53  * @c: UBIFS file-system description object
 54  * @nr_to_write: how many dirty pages to write-back
 55  *
 56  * This function shrinks UBIFS liability by means of writing back some amount
 57  * of dirty inodes and their pages.
 58  *
 59  * Note, this function synchronizes even VFS inodes which are locked
 60  * (@i_mutex) by the caller of the budgeting function, because write-back does
 61  * not touch @i_mutex.
 62  */
 63 static void shrink_liability(struct ubifs_info *c, int nr_to_write)
 64 {
 65         down_read(&c->vfs_sb->s_umount);
 66         writeback_inodes_sb(c->vfs_sb, WB_REASON_FS_FREE_SPACE);
 67         up_read(&c->vfs_sb->s_umount);
 68 }
 69 
 70 /**
 71  * run_gc - run garbage collector.
 72  * @c: UBIFS file-system description object
 73  *
 74  * This function runs garbage collector to make some more free space. Returns
 75  * zero if a free LEB has been produced, %-EAGAIN if commit is required, and a
 76  * negative error code in case of failure.
 77  */
 78 static int run_gc(struct ubifs_info *c)
 79 {
 80         int err, lnum;
 81 
 82         /* Make some free space by garbage-collecting dirty space */
 83         down_read(&c->commit_sem);
 84         lnum = ubifs_garbage_collect(c, 1);
 85         up_read(&c->commit_sem);
 86         if (lnum < 0)
 87                 return lnum;
 88 
 89         /* GC freed one LEB, return it to lprops */
 90         dbg_budg("GC freed LEB %d", lnum);
 91         err = ubifs_return_leb(c, lnum);
 92         if (err)
 93                 return err;
 94         return 0;
 95 }
 96 
 97 /**
 98  * get_liability - calculate current liability.
 99  * @c: UBIFS file-system description object
100  *
101  * This function calculates and returns current UBIFS liability, i.e. the
102  * amount of bytes UBIFS has "promised" to write to the media.
103  */
104 static long long get_liability(struct ubifs_info *c)
105 {
106         long long liab;
107 
108         spin_lock(&c->space_lock);
109         liab = c->bi.idx_growth + c->bi.data_growth + c->bi.dd_growth;
110         spin_unlock(&c->space_lock);
111         return liab;
112 }
113 
114 /**
115  * make_free_space - make more free space on the file-system.
116  * @c: UBIFS file-system description object
117  *
118  * This function is called when an operation cannot be budgeted because there
119  * is supposedly no free space. But in most cases there is some free space:
120  *   o budgeting is pessimistic, so it always budgets more than it is actually
121  *     needed, so shrinking the liability is one way to make free space - the
122  *     cached data will take less space then it was budgeted for;
123  *   o GC may turn some dark space into free space (budgeting treats dark space
124  *     as not available);
125  *   o commit may free some LEB, i.e., turn freeable LEBs into free LEBs.
126  *
127  * So this function tries to do the above. Returns %-EAGAIN if some free space
128  * was presumably made and the caller has to re-try budgeting the operation.
129  * Returns %-ENOSPC if it couldn't do more free space, and other negative error
130  * codes on failures.
131  */
132 static int make_free_space(struct ubifs_info *c)
133 {
134         int err, retries = 0;
135         long long liab1, liab2;
136 
137         do {
138                 liab1 = get_liability(c);
139                 /*
140                  * We probably have some dirty pages or inodes (liability), try
141                  * to write them back.
142                  */
143                 dbg_budg("liability %lld, run write-back", liab1);
144                 shrink_liability(c, NR_TO_WRITE);
145 
146                 liab2 = get_liability(c);
147                 if (liab2 < liab1)
148                         return -EAGAIN;
149 
150                 dbg_budg("new liability %lld (not shrunk)", liab2);
151 
152                 /* Liability did not shrink again, try GC */
153                 dbg_budg("Run GC");
154                 err = run_gc(c);
155                 if (!err)
156                         return -EAGAIN;
157 
158                 if (err != -EAGAIN && err != -ENOSPC)
159                         /* Some real error happened */
160                         return err;
161 
162                 dbg_budg("Run commit (retries %d)", retries);
163                 err = ubifs_run_commit(c);
164                 if (err)
165                         return err;
166         } while (retries++ < MAX_MKSPC_RETRIES);
167 
168         return -ENOSPC;
169 }
170 
171 /**
172  * ubifs_calc_min_idx_lebs - calculate amount of LEBs for the index.
173  * @c: UBIFS file-system description object
174  *
175  * This function calculates and returns the number of LEBs which should be kept
176  * for index usage.
177  */
178 int ubifs_calc_min_idx_lebs(struct ubifs_info *c)
179 {
180         int idx_lebs;
181         long long idx_size;
182 
183         idx_size = c->bi.old_idx_sz + c->bi.idx_growth + c->bi.uncommitted_idx;
184         /* And make sure we have thrice the index size of space reserved */
185         idx_size += idx_size << 1;
186         /*
187          * We do not maintain 'old_idx_size' as 'old_idx_lebs'/'old_idx_bytes'
188          * pair, nor similarly the two variables for the new index size, so we
189          * have to do this costly 64-bit division on fast-path.
190          */
191         idx_lebs = div_u64(idx_size + c->idx_leb_size - 1, c->idx_leb_size);
192         /*
193          * The index head is not available for the in-the-gaps method, so add an
194          * extra LEB to compensate.
195          */
196         idx_lebs += 1;
197         if (idx_lebs < MIN_INDEX_LEBS)
198                 idx_lebs = MIN_INDEX_LEBS;
199         return idx_lebs;
200 }
201 
202 /**
203  * ubifs_calc_available - calculate available FS space.
204  * @c: UBIFS file-system description object
205  * @min_idx_lebs: minimum number of LEBs reserved for the index
206  *
207  * This function calculates and returns amount of FS space available for use.
208  */
209 long long ubifs_calc_available(const struct ubifs_info *c, int min_idx_lebs)
210 {
211         int subtract_lebs;
212         long long available;
213 
214         available = c->main_bytes - c->lst.total_used;
215 
216         /*
217          * Now 'available' contains theoretically available flash space
218          * assuming there is no index, so we have to subtract the space which
219          * is reserved for the index.
220          */
221         subtract_lebs = min_idx_lebs;
222 
223         /* Take into account that GC reserves one LEB for its own needs */
224         subtract_lebs += 1;
225 
226         /*
227          * The GC journal head LEB is not really accessible. And since
228          * different write types go to different heads, we may count only on
229          * one head's space.
230          */
231         subtract_lebs += c->jhead_cnt - 1;
232 
233         /* We also reserve one LEB for deletions, which bypass budgeting */
234         subtract_lebs += 1;
235 
236         available -= (long long)subtract_lebs * c->leb_size;
237 
238         /* Subtract the dead space which is not available for use */
239         available -= c->lst.total_dead;
240 
241         /*
242          * Subtract dark space, which might or might not be usable - it depends
243          * on the data which we have on the media and which will be written. If
244          * this is a lot of uncompressed or not-compressible data, the dark
245          * space cannot be used.
246          */
247         available -= c->lst.total_dark;
248 
249         /*
250          * However, there is more dark space. The index may be bigger than
251          * @min_idx_lebs. Those extra LEBs are assumed to be available, but
252          * their dark space is not included in total_dark, so it is subtracted
253          * here.
254          */
255         if (c->lst.idx_lebs > min_idx_lebs) {
256                 subtract_lebs = c->lst.idx_lebs - min_idx_lebs;
257                 available -= subtract_lebs * c->dark_wm;
258         }
259 
260         /* The calculations are rough and may end up with a negative number */
261         return available > 0 ? available : 0;
262 }
263 
264 /**
265  * can_use_rp - check whether the user is allowed to use reserved pool.
266  * @c: UBIFS file-system description object
267  *
268  * UBIFS has so-called "reserved pool" which is flash space reserved
269  * for the superuser and for uses whose UID/GID is recorded in UBIFS superblock.
270  * This function checks whether current user is allowed to use reserved pool.
271  * Returns %1  current user is allowed to use reserved pool and %0 otherwise.
272  */
273 static int can_use_rp(struct ubifs_info *c)
274 {
275         if (uid_eq(current_fsuid(), c->rp_uid) || capable(CAP_SYS_RESOURCE) ||
276             (!gid_eq(c->rp_gid, GLOBAL_ROOT_GID) && in_group_p(c->rp_gid)))
277                 return 1;
278         return 0;
279 }
280 
281 /**
282  * do_budget_space - reserve flash space for index and data growth.
283  * @c: UBIFS file-system description object
284  *
285  * This function makes sure UBIFS has enough free LEBs for index growth and
286  * data.
287  *
288  * When budgeting index space, UBIFS reserves thrice as many LEBs as the index
289  * would take if it was consolidated and written to the flash. This guarantees
290  * that the "in-the-gaps" commit method always succeeds and UBIFS will always
291  * be able to commit dirty index. So this function basically adds amount of
292  * budgeted index space to the size of the current index, multiplies this by 3,
293  * and makes sure this does not exceed the amount of free LEBs.
294  *
295  * Notes about @c->bi.min_idx_lebs and @c->lst.idx_lebs variables:
296  * o @c->lst.idx_lebs is the number of LEBs the index currently uses. It might
297  *    be large, because UBIFS does not do any index consolidation as long as
298  *    there is free space. IOW, the index may take a lot of LEBs, but the LEBs
299  *    will contain a lot of dirt.
300  * o @c->bi.min_idx_lebs is the number of LEBS the index presumably takes. IOW,
301  *    the index may be consolidated to take up to @c->bi.min_idx_lebs LEBs.
302  *
303  * This function returns zero in case of success, and %-ENOSPC in case of
304  * failure.
305  */
306 static int do_budget_space(struct ubifs_info *c)
307 {
308         long long outstanding, available;
309         int lebs, rsvd_idx_lebs, min_idx_lebs;
310 
311         /* First budget index space */
312         min_idx_lebs = ubifs_calc_min_idx_lebs(c);
313 
314         /* Now 'min_idx_lebs' contains number of LEBs to reserve */
315         if (min_idx_lebs > c->lst.idx_lebs)
316                 rsvd_idx_lebs = min_idx_lebs - c->lst.idx_lebs;
317         else
318                 rsvd_idx_lebs = 0;
319 
320         /*
321          * The number of LEBs that are available to be used by the index is:
322          *
323          *    @c->lst.empty_lebs + @c->freeable_cnt + @c->idx_gc_cnt -
324          *    @c->lst.taken_empty_lebs
325          *
326          * @c->lst.empty_lebs are available because they are empty.
327          * @c->freeable_cnt are available because they contain only free and
328          * dirty space, @c->idx_gc_cnt are available because they are index
329          * LEBs that have been garbage collected and are awaiting the commit
330          * before they can be used. And the in-the-gaps method will grab these
331          * if it needs them. @c->lst.taken_empty_lebs are empty LEBs that have
332          * already been allocated for some purpose.
333          *
334          * Note, @c->idx_gc_cnt is included to both @c->lst.empty_lebs (because
335          * these LEBs are empty) and to @c->lst.taken_empty_lebs (because they
336          * are taken until after the commit).
337          *
338          * Note, @c->lst.taken_empty_lebs may temporarily be higher by one
339          * because of the way we serialize LEB allocations and budgeting. See a
340          * comment in 'ubifs_find_free_space()'.
341          */
342         lebs = c->lst.empty_lebs + c->freeable_cnt + c->idx_gc_cnt -
343                c->lst.taken_empty_lebs;
344         if (unlikely(rsvd_idx_lebs > lebs)) {
345                 dbg_budg("out of indexing space: min_idx_lebs %d (old %d), rsvd_idx_lebs %d",
346                          min_idx_lebs, c->bi.min_idx_lebs, rsvd_idx_lebs);
347                 return -ENOSPC;
348         }
349 
350         available = ubifs_calc_available(c, min_idx_lebs);
351         outstanding = c->bi.data_growth + c->bi.dd_growth;
352 
353         if (unlikely(available < outstanding)) {
354                 dbg_budg("out of data space: available %lld, outstanding %lld",
355                          available, outstanding);
356                 return -ENOSPC;
357         }
358 
359         if (available - outstanding <= c->rp_size && !can_use_rp(c))
360                 return -ENOSPC;
361 
362         c->bi.min_idx_lebs = min_idx_lebs;
363         return 0;
364 }
365 
366 /**
367  * calc_idx_growth - calculate approximate index growth from budgeting request.
368  * @c: UBIFS file-system description object
369  * @req: budgeting request
370  *
371  * For now we assume each new node adds one znode. But this is rather poor
372  * approximation, though.
373  */
374 static int calc_idx_growth(const struct ubifs_info *c,
375                            const struct ubifs_budget_req *req)
376 {
377         int znodes;
378 
379         znodes = req->new_ino + (req->new_page << UBIFS_BLOCKS_PER_PAGE_SHIFT) +
380                  req->new_dent;
381         return znodes * c->max_idx_node_sz;
382 }
383 
384 /**
385  * calc_data_growth - calculate approximate amount of new data from budgeting
386  * request.
387  * @c: UBIFS file-system description object
388  * @req: budgeting request
389  */
390 static int calc_data_growth(const struct ubifs_info *c,
391                             const struct ubifs_budget_req *req)
392 {
393         int data_growth;
394 
395         data_growth = req->new_ino  ? c->bi.inode_budget : 0;
396         if (req->new_page)
397                 data_growth += c->bi.page_budget;
398         if (req->new_dent)
399                 data_growth += c->bi.dent_budget;
400         data_growth += req->new_ino_d;
401         return data_growth;
402 }
403 
404 /**
405  * calc_dd_growth - calculate approximate amount of data which makes other data
406  * dirty from budgeting request.
407  * @c: UBIFS file-system description object
408  * @req: budgeting request
409  */
410 static int calc_dd_growth(const struct ubifs_info *c,
411                           const struct ubifs_budget_req *req)
412 {
413         int dd_growth;
414 
415         dd_growth = req->dirtied_page ? c->bi.page_budget : 0;
416 
417         if (req->dirtied_ino)
418                 dd_growth += c->bi.inode_budget << (req->dirtied_ino - 1);
419         if (req->mod_dent)
420                 dd_growth += c->bi.dent_budget;
421         dd_growth += req->dirtied_ino_d;
422         return dd_growth;
423 }
424 
425 /**
426  * ubifs_budget_space - ensure there is enough space to complete an operation.
427  * @c: UBIFS file-system description object
428  * @req: budget request
429  *
430  * This function allocates budget for an operation. It uses pessimistic
431  * approximation of how much flash space the operation needs. The goal of this
432  * function is to make sure UBIFS always has flash space to flush all dirty
433  * pages, dirty inodes, and dirty znodes (liability). This function may force
434  * commit, garbage-collection or write-back. Returns zero in case of success,
435  * %-ENOSPC if there is no free space and other negative error codes in case of
436  * failures.
437  */
438 int ubifs_budget_space(struct ubifs_info *c, struct ubifs_budget_req *req)
439 {
440         int err, idx_growth, data_growth, dd_growth, retried = 0;
441 
442         ubifs_assert(c, req->new_page <= 1);
443         ubifs_assert(c, req->dirtied_page <= 1);
444         ubifs_assert(c, req->new_dent <= 1);
445         ubifs_assert(c, req->mod_dent <= 1);
446         ubifs_assert(c, req->new_ino <= 1);
447         ubifs_assert(c, req->new_ino_d <= UBIFS_MAX_INO_DATA);
448         ubifs_assert(c, req->dirtied_ino <= 4);
449         ubifs_assert(c, req->dirtied_ino_d <= UBIFS_MAX_INO_DATA * 4);
450         ubifs_assert(c, !(req->new_ino_d & 7));
451         ubifs_assert(c, !(req->dirtied_ino_d & 7));
452 
453         data_growth = calc_data_growth(c, req);
454         dd_growth = calc_dd_growth(c, req);
455         if (!data_growth && !dd_growth)
456                 return 0;
457         idx_growth = calc_idx_growth(c, req);
458 
459 again:
460         spin_lock(&c->space_lock);
461         ubifs_assert(c, c->bi.idx_growth >= 0);
462         ubifs_assert(c, c->bi.data_growth >= 0);
463         ubifs_assert(c, c->bi.dd_growth >= 0);
464 
465         if (unlikely(c->bi.nospace) && (c->bi.nospace_rp || !can_use_rp(c))) {
466                 dbg_budg("no space");
467                 spin_unlock(&c->space_lock);
468                 return -ENOSPC;
469         }
470 
471         c->bi.idx_growth += idx_growth;
472         c->bi.data_growth += data_growth;
473         c->bi.dd_growth += dd_growth;
474 
475         err = do_budget_space(c);
476         if (likely(!err)) {
477                 req->idx_growth = idx_growth;
478                 req->data_growth = data_growth;
479                 req->dd_growth = dd_growth;
480                 spin_unlock(&c->space_lock);
481                 return 0;
482         }
483 
484         /* Restore the old values */
485         c->bi.idx_growth -= idx_growth;
486         c->bi.data_growth -= data_growth;
487         c->bi.dd_growth -= dd_growth;
488         spin_unlock(&c->space_lock);
489 
490         if (req->fast) {
491                 dbg_budg("no space for fast budgeting");
492                 return err;
493         }
494 
495         err = make_free_space(c);
496         cond_resched();
497         if (err == -EAGAIN) {
498                 dbg_budg("try again");
499                 goto again;
500         } else if (err == -ENOSPC) {
501                 if (!retried) {
502                         retried = 1;
503                         dbg_budg("-ENOSPC, but anyway try once again");
504                         goto again;
505                 }
506                 dbg_budg("FS is full, -ENOSPC");
507                 c->bi.nospace = 1;
508                 if (can_use_rp(c) || c->rp_size == 0)
509                         c->bi.nospace_rp = 1;
510                 smp_wmb();
511         } else
512                 ubifs_err(c, "cannot budget space, error %d", err);
513         return err;
514 }
515 
516 /**
517  * ubifs_release_budget - release budgeted free space.
518  * @c: UBIFS file-system description object
519  * @req: budget request
520  *
521  * This function releases the space budgeted by 'ubifs_budget_space()'. Note,
522  * since the index changes (which were budgeted for in @req->idx_growth) will
523  * only be written to the media on commit, this function moves the index budget
524  * from @c->bi.idx_growth to @c->bi.uncommitted_idx. The latter will be zeroed
525  * by the commit operation.
526  */
527 void ubifs_release_budget(struct ubifs_info *c, struct ubifs_budget_req *req)
528 {
529         ubifs_assert(c, req->new_page <= 1);
530         ubifs_assert(c, req->dirtied_page <= 1);
531         ubifs_assert(c, req->new_dent <= 1);
532         ubifs_assert(c, req->mod_dent <= 1);
533         ubifs_assert(c, req->new_ino <= 1);
534         ubifs_assert(c, req->new_ino_d <= UBIFS_MAX_INO_DATA);
535         ubifs_assert(c, req->dirtied_ino <= 4);
536         ubifs_assert(c, req->dirtied_ino_d <= UBIFS_MAX_INO_DATA * 4);
537         ubifs_assert(c, !(req->new_ino_d & 7));
538         ubifs_assert(c, !(req->dirtied_ino_d & 7));
539         if (!req->recalculate) {
540                 ubifs_assert(c, req->idx_growth >= 0);
541                 ubifs_assert(c, req->data_growth >= 0);
542                 ubifs_assert(c, req->dd_growth >= 0);
543         }
544 
545         if (req->recalculate) {
546                 req->data_growth = calc_data_growth(c, req);
547                 req->dd_growth = calc_dd_growth(c, req);
548                 req->idx_growth = calc_idx_growth(c, req);
549         }
550 
551         if (!req->data_growth && !req->dd_growth)
552                 return;
553 
554         c->bi.nospace = c->bi.nospace_rp = 0;
555         smp_wmb();
556 
557         spin_lock(&c->space_lock);
558         c->bi.idx_growth -= req->idx_growth;
559         c->bi.uncommitted_idx += req->idx_growth;
560         c->bi.data_growth -= req->data_growth;
561         c->bi.dd_growth -= req->dd_growth;
562         c->bi.min_idx_lebs = ubifs_calc_min_idx_lebs(c);
563 
564         ubifs_assert(c, c->bi.idx_growth >= 0);
565         ubifs_assert(c, c->bi.data_growth >= 0);
566         ubifs_assert(c, c->bi.dd_growth >= 0);
567         ubifs_assert(c, c->bi.min_idx_lebs < c->main_lebs);
568         ubifs_assert(c, !(c->bi.idx_growth & 7));
569         ubifs_assert(c, !(c->bi.data_growth & 7));
570         ubifs_assert(c, !(c->bi.dd_growth & 7));
571         spin_unlock(&c->space_lock);
572 }
573 
574 /**
575  * ubifs_convert_page_budget - convert budget of a new page.
576  * @c: UBIFS file-system description object
577  *
578  * This function converts budget which was allocated for a new page of data to
579  * the budget of changing an existing page of data. The latter is smaller than
580  * the former, so this function only does simple re-calculation and does not
581  * involve any write-back.
582  */
583 void ubifs_convert_page_budget(struct ubifs_info *c)
584 {
585         spin_lock(&c->space_lock);
586         /* Release the index growth reservation */
587         c->bi.idx_growth -= c->max_idx_node_sz << UBIFS_BLOCKS_PER_PAGE_SHIFT;
588         /* Release the data growth reservation */
589         c->bi.data_growth -= c->bi.page_budget;
590         /* Increase the dirty data growth reservation instead */
591         c->bi.dd_growth += c->bi.page_budget;
592         /* And re-calculate the indexing space reservation */
593         c->bi.min_idx_lebs = ubifs_calc_min_idx_lebs(c);
594         spin_unlock(&c->space_lock);
595 }
596 
597 /**
598  * ubifs_release_dirty_inode_budget - release dirty inode budget.
599  * @c: UBIFS file-system description object
600  * @ui: UBIFS inode to release the budget for
601  *
602  * This function releases budget corresponding to a dirty inode. It is usually
603  * called when after the inode has been written to the media and marked as
604  * clean. It also causes the "no space" flags to be cleared.
605  */
606 void ubifs_release_dirty_inode_budget(struct ubifs_info *c,
607                                       struct ubifs_inode *ui)
608 {
609         struct ubifs_budget_req req;
610 
611         memset(&req, 0, sizeof(struct ubifs_budget_req));
612         /* The "no space" flags will be cleared because dd_growth is > 0 */
613         req.dd_growth = c->bi.inode_budget + ALIGN(ui->data_len, 8);
614         ubifs_release_budget(c, &req);
615 }
616 
617 /**
618  * ubifs_reported_space - calculate reported free space.
619  * @c: the UBIFS file-system description object
620  * @free: amount of free space
621  *
622  * This function calculates amount of free space which will be reported to
623  * user-space. User-space application tend to expect that if the file-system
624  * (e.g., via the 'statfs()' call) reports that it has N bytes available, they
625  * are able to write a file of size N. UBIFS attaches node headers to each data
626  * node and it has to write indexing nodes as well. This introduces additional
627  * overhead, and UBIFS has to report slightly less free space to meet the above
628  * expectations.
629  *
630  * This function assumes free space is made up of uncompressed data nodes and
631  * full index nodes (one per data node, tripled because we always allow enough
632  * space to write the index thrice).
633  *
634  * Note, the calculation is pessimistic, which means that most of the time
635  * UBIFS reports less space than it actually has.
636  */
637 long long ubifs_reported_space(const struct ubifs_info *c, long long free)
638 {
639         int divisor, factor, f;
640 
641         /*
642          * Reported space size is @free * X, where X is UBIFS block size
643          * divided by UBIFS block size + all overhead one data block
644          * introduces. The overhead is the node header + indexing overhead.
645          *
646          * Indexing overhead calculations are based on the following formula:
647          * I = N/(f - 1) + 1, where I - number of indexing nodes, N - number
648          * of data nodes, f - fanout. Because effective UBIFS fanout is twice
649          * as less than maximum fanout, we assume that each data node
650          * introduces 3 * @c->max_idx_node_sz / (@c->fanout/2 - 1) bytes.
651          * Note, the multiplier 3 is because UBIFS reserves thrice as more space
652          * for the index.
653          */
654         f = c->fanout > 3 ? c->fanout >> 1 : 2;
655         factor = UBIFS_BLOCK_SIZE;
656         divisor = UBIFS_MAX_DATA_NODE_SZ;
657         divisor += (c->max_idx_node_sz * 3) / (f - 1);
658         free *= factor;
659         return div_u64(free, divisor);
660 }
661 
662 /**
663  * ubifs_get_free_space_nolock - return amount of free space.
664  * @c: UBIFS file-system description object
665  *
666  * This function calculates amount of free space to report to user-space.
667  *
668  * Because UBIFS may introduce substantial overhead (the index, node headers,
669  * alignment, wastage at the end of LEBs, etc), it cannot report real amount of
670  * free flash space it has (well, because not all dirty space is reclaimable,
671  * UBIFS does not actually know the real amount). If UBIFS did so, it would
672  * bread user expectations about what free space is. Users seem to accustomed
673  * to assume that if the file-system reports N bytes of free space, they would
674  * be able to fit a file of N bytes to the FS. This almost works for
675  * traditional file-systems, because they have way less overhead than UBIFS.
676  * So, to keep users happy, UBIFS tries to take the overhead into account.
677  */
678 long long ubifs_get_free_space_nolock(struct ubifs_info *c)
679 {
680         int rsvd_idx_lebs, lebs;
681         long long available, outstanding, free;
682 
683         ubifs_assert(c, c->bi.min_idx_lebs == ubifs_calc_min_idx_lebs(c));
684         outstanding = c->bi.data_growth + c->bi.dd_growth;
685         available = ubifs_calc_available(c, c->bi.min_idx_lebs);
686 
687         /*
688          * When reporting free space to user-space, UBIFS guarantees that it is
689          * possible to write a file of free space size. This means that for
690          * empty LEBs we may use more precise calculations than
691          * 'ubifs_calc_available()' is using. Namely, we know that in empty
692          * LEBs we would waste only @c->leb_overhead bytes, not @c->dark_wm.
693          * Thus, amend the available space.
694          *
695          * Note, the calculations below are similar to what we have in
696          * 'do_budget_space()', so refer there for comments.
697          */
698         if (c->bi.min_idx_lebs > c->lst.idx_lebs)
699                 rsvd_idx_lebs = c->bi.min_idx_lebs - c->lst.idx_lebs;
700         else
701                 rsvd_idx_lebs = 0;
702         lebs = c->lst.empty_lebs + c->freeable_cnt + c->idx_gc_cnt -
703                c->lst.taken_empty_lebs;
704         lebs -= rsvd_idx_lebs;
705         available += lebs * (c->dark_wm - c->leb_overhead);
706 
707         if (available > outstanding)
708                 free = ubifs_reported_space(c, available - outstanding);
709         else
710                 free = 0;
711         return free;
712 }
713 
714 /**
715  * ubifs_get_free_space - return amount of free space.
716  * @c: UBIFS file-system description object
717  *
718  * This function calculates and returns amount of free space to report to
719  * user-space.
720  */
721 long long ubifs_get_free_space(struct ubifs_info *c)
722 {
723         long long free;
724 
725         spin_lock(&c->space_lock);
726         free = ubifs_get_free_space_nolock(c);
727         spin_unlock(&c->space_lock);
728 
729         return free;
730 }
731 

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